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Old 29th March 2011, 09:05 AM   #1
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Default Passive Preamp - Grounding plan

Hi,

Iíve searched and read a lot about this, but Iím struggling to find a definitive answer for my particular case.

How does this sound for a grounding plan:

The input selector only switches signal +ve, sends it to the stepped attenuator, then on to the output +ve. The left input grounds are bussed together, along with the left output ground. The right input grounds are bussed together, along with the right output ground. This keeps the left and right grounds and signals completely separate.

Will this work, or do I need to connect these ground busses to the chassis (aluminium) somewhere to avoid RFI? Also, in this arrangement each signal ground is connected to the ground circuits of all the other pieces of equipment currently plugged in to the inputs. Will this be a nightmare for hum? I canít see an alternative to this though, as I canít have a separate ground for every RCA connector unless I double up on poles and switch the grounds as well as the +ve. Almost every design of passive pre Iíve seen doesnít switch grounds though, only the +ve. Confused.

Cheers,
Leon
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Old 29th March 2011, 11:21 AM   #2
AndrewT is offline AndrewT  Scotland
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anything more than monoblock construction requires signal grounds and power grounds to be interconnected at some point.

Read
Audio Component Grounding and Interconnection - diyAudio
thoroughly and go back and pick out the important messages for your intended use.

Only 58k views. There's a lot of Members that refuse to research.
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Old 29th March 2011, 12:14 PM   #3
DF96 is offline DF96  England
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If a 'passive preamp' has a metal box then the box has to be grounded, otherwise it could increase pickup.
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Old 29th March 2011, 12:36 PM   #4
AndrewT is offline AndrewT  Scotland
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A shield does not need to be grounded. Or am I wrong again?
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Old 29th March 2011, 01:40 PM   #5
DF96 is offline DF96  England
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Inside a screen the potential is everywhere the same. A circuit completely inside the screen, with no external connections, will not be aware of any electric fields outside the screen. In this situation the screen does not need grounding.

If the circuit has an external connection (and most do) then it becomes sensitive to the difference between its ground and the screen potential. An ungrounded screen then becomes a probe, sensing the external field and passing it on to the internal circuit. As the screen is presumably larger than the circuit, it will pick up interference more effectively than the circuit would on its own because it has a larger capacitance to any external conductor.
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Old 29th March 2011, 02:15 PM   #6
AndrewT is offline AndrewT  Scotland
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Why/How does your explanation confirm that the screen (in this situation metal chassis) must be grounded?
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Old 29th March 2011, 02:23 PM   #7
DF96 is offline DF96  England
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The circuit inside has external connections. In this case, another name for an ungrounded screen is a coupling capacitor!
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Old 29th March 2011, 02:30 PM   #8
AndrewT is offline AndrewT  Scotland
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I still can't see why?
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Old 29th March 2011, 02:36 PM   #9
pooge is offline pooge  United States
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Quote:
Originally Posted by AndrewT View Post
I still can't see why?
Andrew,

Go back and read the reference you recommended. I don't have a page number, but it's in there.
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Old 29th March 2011, 02:39 PM   #10
AndrewT is offline AndrewT  Scotland
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I happen to have it open since earlier this morning.
I will go and look again.
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